1. Field of the Invention 1Technical Field
The present invention relates to polyphenylene ether (PPE) microspore dispersions, and more particularly to a process for preparing a PPE microspore dispersion that is suitable for impregnation processes performed below 40° C. 2. Description of Related Art
PPE (Polyphenylene Ether) represents a resin featuring for excellent insulating capability, acid/alkaline resistance and superior Dk (dielectric constant) as well as Df (dielectric dissipation factor). This material is also suitable for high-frequency copper clad laminates that require low Dk and low Df, and thus has been extensively used in high-frequency communication devices.
PPE resins have better electrical properties when the molecular weight is high. For good electrical properties, each molecule of polyphenylene ether (PPE) perferably has an average phenolic hydroxyl groups (i.e., number of OH groups) ranging between 0.001 and 0.1. However, PPE resins currently used for making high-frequency copper clad laminates (HF-CCLs) are mostly low-molecular weight PPE (hereinafter abbreviated as LR-PPE) that have a number-average molecular weight (Mn) smaller than 9,000 g/mole. While LR-PPE has the advantages of being highly dissolvable in solvents, and suitable for impregnation below 40° C., it is considerably inferior to high-molecular weight PPE (hereinafter abbreviated as HM-PPE) having a number-average molecular weight (Mn) greater than 12,000 g/mole in terms of electrical property, glass transition temperature (Tg) and mechanical strength, and has no help in improving high-frequency copper clad laminates.
In the conventional method that uses PPE resins to make high-frequency copper clad laminates, one of methylbenzene, butanone and dimethylformamide is used as a solvent to dissolve PPE and form a PPE solution. Then processing aids (or called auxiliary agents) such as flame retardants, silica, binding agents, or initiators are added and well agitated to form varnish. The varnish is later used to impregnate glass fabrics at 40° C. After impregnation varnish, the prepreg is dried and undergoes heat press to be made into high-frequency copper clad laminates.
For endowing high-frequency copper clad laminates with improved electrical properties, PPE resins used in HF-CCL manufacturing are perferably HM-PPE resins, namely those having a number-average molecular weight (Mn) greater than 12,000 g/mole. During preparation of varnish and during impregnation, however, the HM-PPE solution requires a temperature higher than 40° C. The requirement for high-temperature impregnation brings about the need of high-temperature impregnation equipment for making high-frequency copper clad laminates. This in turn leads to problems relating high costs and processing safety.
As an approach to addressing theses issues, HM-PPE resins having a number-average molecular weight (Mn) larger than 8,000 but less than 40,000 are now ground, smashed or crystalized into HM-PPE particles. By using varnish made of these HM-PPE particles evenly mixed with specific processing aids required, it is possible to perform impregnation at the room temperature. However, such a PPE dispersion only allows the HM-PPE particles to be stayed with the processing aids and then causes both finally becomes like a solid-to-solid contact or a solid-to-liquid contact, so that such a PPE dispersion is contrary to a real microspore dispersion defined with structure of having HM-PPE microspores fully wrapped the processing aids.
Thus, the existing PPE dispersion disclosed in the state of art is quite not homogenized, if observed in view of micro phase. Such a non-homogenous PPE dispersion for use in making HF-CCLs has negative impact to the resultant HF-CCLs in terms of electrical property, mechanical strength and heat resistance.